As an analyzer for analyzing inorganic elements with high precision, a plasma mass spectrometer is known. This instrument introduces an atomized sample to be analyzed into plasma formed over a plasma torch; ionizes elements contained in the sample; extracts ions present in the plasma in the form of an ion beam; and conducts mass spectrum analysis on ions forming the ion beam. As plasma to which a sample is introduced, used is inductively-coupled plasma (ICP) generated using as an energy source a high frequency electromagnetic field provided from a coil adjacent to a plasma torch; or microwave plasma generated by a microwave introduced to a tip of a plasma torch. In general, the former is known as an inductively-coupled plasma mass spectrometer (ICP-MS) and the latter is known as a microwave induced plasma mass spectrometer (MIP-MS).
FIG. 7 is a schematic view showing a basic concept of an exemplary inductively-coupled plasma mass spectrometer (hereinafter, also referred to simply as instrument) 11 according to the conventional art. The instrument 11 has a plasma torch 20 for generating plasma 22, an interface section 30 placed at a position facing the plasma 22, an ion lens section 50 placed behind the interface section 30, an ion guide section 70 placed behind the ion lens section 50, and a mass analysis section 80 placed behind the ion guide section 70. The instrument 11 can generally measure positive ions, but it can also measure negative ions. This specification is described under the assumption that the device 11 measures positive ions. It is evident to those skilled in the art that when the instrument 11 measures negative ions, the polarity of a voltage to be applied to an electrode or the like is inverted.
The plasma torch 20 has a coil 21 for generating a high frequency electromagnetic field near its tip, and is placed under atmospheric pressure. The coil 21 is connected to an RF power source not illustrated. In the plasma torch 20, the high frequency electromagnetic field generated by the coil 21 produces high frequency inductively-coupled plasma 22. In the plasma torch 20, an atomized sample not illustrated is introduced into the plasma 22 from the front of the plasma torch 20. The introduced sample not illustrated is vaporized and decomposed by the action of the plasma 22; and in cases of large majority of elements, they are finally converted into ions. The ionized sample not illustrated is contained in the plasma 22. Further, within the plasma torch 20, a gas flow occurs from the back end to the front end, so the plasma 22 extends towards a sampling cone 31.
The interface section 30 is provided with two cone members, that is the sampling cone 31 and a skimmer cone 33. A part of plasma 32 having passed through an aperture 37 of the sampling cone 31 directly facing the plasma 22 reaches the skimmer cone 33 positioned further behind. Thereafter, a part of plasma 32 passes through an aperture 38 formed in the skimmer cone 33 and reaches the rear thereof. Gas molecules (including neutralized ions) not having passed through the skimmer cone 33 are discharged from the interface section 30 via an exhaust port 39 by a rotary pump RP.
The ion lens section 50 is provided with a first electrode 53 and a second electrode 54 forming an extraction electrode section. The first electrode 53 or the second electrode 54 forming the extraction electrode section is at negative potential, and thus, only positive ions are extracted from the plasma 52 in the form of an ion beam. The ion beam is guided from the second electrode 54 into a collision/reaction cell 71 of the ion guide section 70. However, an ion deflection lens is arranged subsequent to the second electrode 54 and the ion beam may be guided into the collision/reaction cell 71 via the ion deflection lens.
The ion beam guided into the collision/reaction cell 71 is induced to a subsequent stage along a track determined by an electric field generated by a multipole electrode 73. The multipole electrode 73 has, for example, an octapole structure. Further, a collision/reaction gas may be introduced from a feeding port 72 into the collision/reaction cell 71. Molecules of the introduced gas cause reaction associated with collision or charge transfer with various ions contained in the ion beam, thereby removing, from the ion beam, polyatomic ions or interference ions that are composed of elements contained in carrier gas and the sample and cause interferences in mass spectra.
During operation of the instrument 11, the ion guide section 70 is exhausted together with the ion lens section 50 by using a turbo molecular pump (TMP1). Therefore, molecules that have been contained in the plasma but neutralized within the ion lens section 50 or the ion guide section 70, or molecules of collision/reaction gas that are introduced into the collision/reaction cell are exhausted through an exhaust port 79.
An ion beam 75 out of the collision/reaction cell 71 is introduced into the mass analysis section 80. In the mass analysis section 80, there is provided a multipole structure 81 of quadrupole, which is known as a quadrupole mass analyzer or a quadrupole mass filter (hereinafter, the multipole structure 81 is referred to as a mass analyzer). An electric field generated by the mass analyzer allows ions in the ion beam to pass through the mass analyzer 81 along an X-axis in FIG. 7 and to be separated based on a mass-to-charge ratio. Subsequently, separated ions 85 (indicated by a broken line) are guided to a subsequent ion detector 82. The mass analysis section 80 is also exhausted by using a turbo molecular pump (TMP2) in the same manner as the ion guide section 70, and unnecessary ions separated by the mass analyzer 81 and other molecules are exhausted through an exhaust port 84.
The ion detector 82 receives and detects ions separated at the mass analyzer 81 to convert into electric signals. For example, an inductively-coupled plasma mass spectrometer (ICP-MS) is an instrument having a large dynamic range to detect from signals for trace quantities (e.g., 0.1 cps) to signals for main components (e.g., 1010 cps). In general, when detected signals are low, ion counting is used for measurement; and when detected signals are high, analog measurement is used. For example, in the case of ion counting, ions are introduced into a secondary electron multiplier thereby to be converted to 105 to 106-times amplified electrons. Such electrons are converted into a voltage pulse and counted for a certain period of time and thereby, an ion count is obtained.
In such a mass spectrometer, when ions are extracted from plasma at the first electrode 53 or the second electrode 54, neutral particles with high energy are produced. Such neutral particles are generally known as a cause for background noises, and separation of these neutral particles from ions is required. Mechanisms for conducting such separation are disclosed, for example, in Patent Document 1 (Japanese Patent Laid-Open Publication No. H7-78,590); Patent Document 2 (National Publication of International Patent Application No. 2002-525,821); and Patent Document 3 (National Publication of International Patent Application No. 2004-515,882).
Patent Document 1, for example, discloses that an ion lens has a 90° deflector, whereby neutral particles contained in an ion beam having passed through an interface are prevented from reaching a mass filter. Further, Patent Document 2 discloses that a beam composed of ions and neutral particles coming through an opening of a skimmer cone is reflected at 90° by an ion mirror and sent to a mass analyzer, whereby neutral particles are prevented from reaching the mass analyzer.
Patent Document 3 discloses an ion mirror 42 similar to that of Patent Document 2. In order to increase the transmission of an ion injection port of a mass analysis section, Patent Document 3 also discloses that quadrupole fringe electrodes 56 are provided between the ion mirror 42 and a linear quadrupole mass analyzer 54. Four rod-shaped electrodes of this quadrupole fringe electrode 56 are curved while being kept parallel to each other, and prevent neutral particles from reaching the linear quadrupole mass analyzer 54.
However, when ions are introduced into a mass analyzer (e.g., quadrupole mass analyzer); and these ions are accelerated by an RF voltage of quadrupole electrodes and collided with molecules of residual gas, the ions may be changed to neutral particles having energy before the collision. These neutral particles collide with a wall near within an ion detector thereby to generate secondary ions, which may be detected as background noises by the ion detector. In particular, a plasma mass spectrometer has a larger amount of ions derived from carrier gas than GC-MS or LC-MS. Thus, it is likely to have a drawback on background noises caused by the generation of neutral particles.
Further, when a deflector or an ion mirror is arranged prior to a mass analyzer as disclosed in Patent Documents 1 to 3, a certain amount of ions to be measured is lost and the measurement sensitivity may be deteriorated. This is because a difference of deflection angle occurs due to the energy difference depending on the mass number of an ion; or a difference in the output position of an ion due to the incident position or the incident angle of the ion to a deflector. In addition, curved quadrupole fringe electrodes disclosed in Patent Document 3 may have a reduced ion transmission in comparison with a simple straight fringe electrode. Curving four rod-shaped electrodes while keeping them parallel to each other would result in a complicated structure and increase the cost and labor for processing.